In a continual effort to reduce weight and improve the fuel efficiency of automotive vehicles and the like, engineers seek to substitute lighter (and usually softer) materials such as aluminum and magnesium alloys for the more dense and more scuff- and wear-resistant iron and steel alloys that are widely used. Often the substitution of aluminum in engines and other powertrain component applications exposes them to sliding contact with other aluminum parts. While aluminum alloy parts are strong, they are susceptible to wear and scuffing when they rub on each other. Their surface properties are such that they must be adequately lubricated in such applications. For example, it is desirable to employ aluminum alloy pistons in an aluminum alloy engine block. In normal warmed-up engine operation, a film of oil can be maintained between each reciprocating piston and its cylinder wall. However, during cold start and other situations, if the oil film has not been established, aluminum surfaces in sliding engagement will soon scuff. Material from the piston or the engine block surface is transferred to the opposite member, which causes additional scuffing and tearing and failure of the parts.
Thus, where aluminum alloy pistons and engine blocks are used in automotive engine applications, it is the practice to provide iron liners in the cylinder walls of the aluminum block. The aluminum pistons then engage iron liners, and even during cold starts, no scuffing or undue wear occurs. However, the manufacture and placement of iron liners in the cylinder bores of an aluminum engine block represent an undesirable additional expense and weight.
The properties of certain amorphous hydrogen-containing carbon deposits have been investigated as protective layers on surfaces that are to be subjected to wear or scuffing. When suitably formed, these carbonaceous materials are found to be extremely hard and wear resistant, and they are sometimes called diamond-like carbon. Once recognized, it has been suggested that such coatings could be used on aluminum alloy components in sliding engagement with other aluminum alloy components in automotive powertrain environments. See, for example, U.S. Pat. Nos. 5,237,967; 5,249,554 and 5,309,874. However, the difficulty in utilizing such hard carbon coatings on automotive components is that the cost of forming adherent and uniform coatings on workpieces of complex shape has been too high. In other words, the processing has been too slow and too cumbersome.
Accordingly, it is an object of the present invention to provide an efficient process of applying adherent, hard and low friction amorphous carbon coatings to the surfaces of three-dimensional workpieces of suitable materials susceptible to carbon ion implantation. The present process is particularly useful on workpieces of aluminum alloys, ferrous metal alloys and other alloys capable of forming intermetallic carbides. It is a further object to provide a method of providing such coatings that strongly adhere to the workpiece substrate. It is a still further object of the present invention to provide a method that is efficient and practical enough for automotive manufacturing requirements.
In accordance with this invention, these and other objects and advantages are accomplished as follows.